Capacitance-type oil level measuring devices work on the principle of detecting changes in electrical capacitance between two insulated metal plates when a liquid is introduced between the plates. Typically, these devices employ a small diameter pipe for one plate of the capacitor inside of a larger pipe that serves as the second plate. An electronic processor connects to the two plates and converts a change in capacitance to a voltage level that is sent to a read out system to indicate the gallons of liquid present. Such a probe is known in the prior art from P/N 74-113-1, manufactured by Meggitt Avionics, Inc., Manchester, N.H.
In use, these probes are mounted on top of an aluminum oil tank with the probe end extending downward into the oil to a location near the bottom of the tank. Such probes typically can measure in a range from about 8 gallons down to 1.5 gallons. The electronics section of the prior art probes is mounted directly on top of the probe, thus transferring any heat from the oil into the electronic components. Electronic components (e.g., transistors, resistors, etc.) are heat sensitive and change characteristics with every incremental change in temperature. The oil in these tanks can reach temperatures of over 100 degrees centigrade. The prior art probes have no thermal compensation and were therefore subject to thermal drift. As a result of this thermal drift when the prior art probe heats up, its output increases, making it appear that more oil is available after running the engine, than was available before running the engine. Clearly, this is an erroneous and undesirable outcome.
Moreover, due to the heat and vibration, such a prior art probe has been known to leak water into its electronics compartment, thereby adversely affecting its operation even more.